In pressurized-water nuclear reactors primary coolant, which is heated by the reactor itself, is sent to a steam generator in which it is employed to heat a secondary coolant, a coolant that is physically but not thermally isolated from the primary coolant. The secondary coolant is typically water, and it is turned to steam by heat from the primary coolant. The steam turns a turbine, which drives an electrical generator. It is the electrical generator that supplies power to the power lines.
It is a matter of some concern to the generator owner that almost no water droplets be carried by the steam into the turbine. Elimination of carryover is important because water droplets can cause quite costly mechanical damage to the turbines. As a result, some of the power companies that purchase steam-supply systems require that some provision be made for shutting down the turbine when it appears that water droplets will be entrained by the steam. Since it is known that there is a positive correlation between high steam-generator water level and water droplet carryover, the solution resorted to by the steam-generator vendor has typically been to measure the water level in the steam generator and trip, or remove the steam supply from, the turbine whenever the water level exceeds a predetermined height.
This system is ordinarily adequate, but it can present inconveniences. This is particularly true when both the turbine and the reactor must be shut down in response to a high-water-level condition. Once a reactor has been shut down, it is a matter of several hours before it can be brought on line again, and each hour of downtime constitutes an enormous expense to the power company.